1. Field of Invention
The present invention relates to a method for manufacturing a semiconductor device. The present invention particularly relates to a method for manufacturing a semiconductor device, in which a semiconductor film is heated by laser irradiation.
2. Description of the Related Art
In recent years, attention has been attracted to a technique for manufacturing thin film transistors (TFTs) by using a semiconductor thin film (with a thickness of about several nanometers to several hundred nanometers) formed over a substrate having an insulating surface. Thin film transistors are widely applied to electronic devices such as ICs and electro-optic devices, and development of thin film transistors as switching elements particularly for image display devices is urgently required.
With the increase in size of image display devices and image sensors and in density of pixels (higher definition), elements using semiconductor thin films capable of following higher-speed driving are required to be developed. Moreover, in order to reduce weight and cost, thin film transistors have come to be used as not only switching elements of image display devices but also driver elements provided around display regions.
In order to fulfill such requirements, researches have been carried out on a technique for improving electrical characteristics such as electric field effect mobility (also called mobility, simply) by forming a semiconductor film having a crystal structure. For example, a solid-phase growth method and a method of carrying out a heat treatment using a laser beam (also called “laser light”) are studied. The method of carrying out a heat treatment using a laser beam is hereinafter referred to as a laser anneal method. In particular, the laser anneal method has advantages in that process time can be drastically shortened as compared with a solid-phase growth method and a semiconductor substrate or a semiconductor film over a substrate can be selectively heated so that the substrate is hardly damaged thermally (for example, see Reference 1: Japanese Patent Laid-Open No. 2003-297751).
Lasers used in the laser anneal method are classified into gas lasers, liquid lasers, and solid lasers according to their mediums. As a laser oscillation method, a continuous wave oscillation type and a pulsed oscillation type are given.
A technique has been attracting attention lately in which a semiconductor film is heated by a solid laser (such as a Nd:YVO4 laser) instead of a method in which a semiconductor film is heated by an excimer laser, which is one kind of gas lasers (hereinafter also referred to as an excimer laser anneal (ELA) method in this specification). Although a solid laser can form a large crystal grain or a single crystal as compared with the excimer laser anneal method, a solid laser has the following drawbacks.
In the case of crystallizing a semiconductor film formed over a large substrate, a long linear beam is required in consideration of throughput. However, since a solid laser has difficulty in forming a long linear beam, mass productivity is low. This is a problem caused in the case of using a solid laser in a process for heating a semiconductor film.
In order to form a long linear beam, it is necessary to increase output power of a laser beam, and moreover, to heat a semiconductor film effectively, i.e., have a semiconductor film absorb a laser beam effectively. A wavelength of a laser beam which has high absorption efficiency to a silicon film, which is a typical semiconductor film, ranges from visible to ultraviolet regions; on the other hand, a wavelength region of a laser beam of a fundamental wave used as a solid laser mainly ranges from infrared to near-infrared regions. Therefore, in order to effectively heat a semiconductor film by using a solid laser, it is necessary to use a non-linear optical element having a function of converting a laser beam of a fundamental wave into a harmonic (such as a second harmonic or a third harmonic). However, this non-linear optical element has a threshold at which the element is damaged by input power of a fundamental wave; therefore, increasing output power of a harmonic has restriction.
Moreover, a solid laser has a complex oscillator which is sensitive to vibration, and further has problems of high power consumption, necessity of an optical system for beam shaping, necessity of maintenance and cooling water, and the like.